To what extent such aggressive rationalism ever spread it is now quite impossible to ascertain. It seems probable that the word Lokâyata, defined by Sanskrit scholars as signifying "directed to the world of sense," [150] originally, or about 500 B.C., signified "Nature-lore," and that this passed as a branch of Brahman learning. [151] Significantly enough, while the lore was not extensive, it came to be regarded as disposing men to unbelief, though it does not seem to have suggested any thorough training. At length, in the eighth century of our era, it is found applied as a term of abuse, in the sense of "infidel," by Kumârila in controversy with opponents as orthodox as himself; and about the same period Sankara connects with it a denial of the existence of a separate and immortal soul; [152] though that opinion had been debated, and not called Lokâyata, long before, when the word was current in the broader sense. [153] Latterly, in the fourteenth century, on the strength of some doggerel verses which cannot have belonged to the early Brahmanic Lokâyata, it stands for extreme atheism and a materialism not professed by any known school speaking for itself. [154] The evidence, such as it is, is preserved only in Sarva-darsana-samgraha, a compendium of all philosophical systems, compiled in the fourteenth century by the Vedantic teacher Mâdhavâchâra. [155] One source speaks of an early text-book of materialism, the Sutras of Brihaspati; [156] but this has not been preserved. Thus in Hindu as in later European freethought for a long period we have had to rely for our knowledge of freethinkers' ideas upon the replies made by their opponents. It is reasonable to conclude that, save insofar as the arguments of Brihaspati were common to the Khârvâkas and the Buddhists, [157] such doctrine as his or that of the later Lokâyatikas cannot conceivably have been more than the revolt of a thoughtful minority against official as well as popular religion; and to speak of a time when "the Aryan settlers in India had arrived at the conviction that all their Devas or Gods were mere names" [158] is to suggest a general evolution of rational thought which can no more have taken place in ancient India than it has done to-day in Europe. The old creeds would always have defenders; and every revolt was sure to incur a reaction. In the Hitopadesa or "Book of Good Counsel" (an undated recension of the earlier Panchatantra, "The Five Books," which in its first form may be placed about the fifth century of our era) there occur both passages disparaging mere study of the Sacred Books [159] and passages insisting upon it as a virtue in itself [160] and otherwise insisting on ritual observances. [161] They seem to come from different hands.